There’s two ways to go about this set the background color or add a border. Either way you’ll want to add the code to your custom.css or Quick CSS.

Set the background color -

.thumbnail_container {
background:#f8f8f8;
padding:5px;
}

.thumbnail_container:hover {
background:#2c3547;
}

This code will apply a background color to the container just like Abundance does. Notice I have two declarations .thumbnail_container and .thumbnail_container:hover. The first one is for the mouseoff or normal state. The second one is for the mouseover or hover state.

Add a border -

.thumbnail_container {
border:5px solid #f8f8f8;
}

.thumbnail_container:hover {
border:5px solid #2c3547;
}

This code will add a border around the image’s container. Again, I’ve include a normal state and hover state. You’ll only need to use .thumbnail_container:hover.